Providing a tow cable having plural electromagnetic receivers and one or more electromagnetic sources

ABSTRACT

To perform marine electromagnetic (EM) surveying of a subterranean structure, a marine cable system is provided including a tow cable, a plurality of electromagnetic (EM) sources coupled to the tow cable, and a plurality of EM receivers coupled to the tow cable. The system is configured for deployment in a body of water to perform marine EM surveying of a subterranean structure.

BACKGROUND

Various electromagnetic techniques exist to perform surveys of asubterranean structure for identifying elements of interest. Examples ofelements of interest in the subterranean structure include subsurfaceresistive bodies, such as hydrocarbon-bearing reservoirs, gas injectionzones, thin carbonate or salt layers, and fresh-water aquifers. Onesurvey technique is the magnetotelluric (MT) survey technique thatemploys time measurements of electric and magnetic fields (which areresponsive to naturally occurring electromagnetic fields) fordetermining the electrical conductivity distribution beneath thesurface. Another survey technique is the controlled sourceelectromagnetic (CSEM) survey technique, in which an electromagnetictransmitter, called a “source,” is used to generate electromagneticsignals. With either survey technique, surveying units, called“receivers,” are deployed on a surface (such as at the sea floor or onland) within an area of interest to make measurements from whichinformation about the subterranean structure can be derived. Thereceivers may include a number of sensors for detecting any combinationof electric fields, electric currents, and magnetic fields.

In marine environment CSEM surveys, modeling and acquisition studieshave shown that thin resistive targets in a subterranean structure, suchas hydrocarbon-bearing reservoirs, gas injection zones, thin carbonateor salt layers, fresh water aquifers, and so forth, are more easilydetectable when a CSEM source is positioned close to the sea floor. Inpractice, the CSEM source is towed, or “flown,” as close to the seafloor as conditions will allow. Typically, the CSEM source will be towedbetween 30 to 50 meters above the sea floor.

Usually, when performing CSEM surveying, EM receivers are placed on thesea floor. An issue associated with deploying EM receivers on the seafloor is that such deployment is both labor and time-intensive. Also,after the surveying is completed, retrieving or recovering the EMreceivers from the sea floor is also a labor and time-intensive process.Moreover, sea floor receivers tend to measure a total EM field thatcontains the response of not only targets of interest, but also theresponse of sea water, and in a shallow water environment, the responseof air above the sea water.

SUMMARY

In one aspect, the invention relates to a marine cable system. Themarine cable system includes a tow cable, a plurality of electromagnetic(EM) sources coupled to the tow cable, and a plurality of EM receiverscoupled to the tow cable. The system is configured for deployment in abody of water to perform marine EM surveying of a subterraneanstructure.

In one aspect, the invention relates to a method of characterizing asubsurface marine environment. The method includes deploying a surveyingassembly in a body of water; said surveying assembly comprising a towcable, one or more streamers coupled to the tow cable, a plurality of EMsources and a plurality of EM receivers coupled to each streamer. Themethod also includes activating at least one of the EM sources. Themethod also includes acquiring measurement data from the EM receivers inresponse to activation of the at least one EM source.

In one aspect, the invention relates to a method of removing unwantedsignal components from a total electromagnetic field measured on a towedmarine EM system. The method includes providing an arrangement of a pairof EM receivers around an EM source. The method also includes receivingmeasurement data at the pair of the EM receivers in response toactivation of the EM source. The method also includes calculating abucking coefficient based on the measurement data, and removing anunwanted signal component at the EM receivers based on the buckingcoefficient.

Other or alternative features will become apparent from the followingdescription, from the drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 illustrate various configurations in accordance withembodiments of the present disclosure that include one or more towcables coupled to streamers having EM sources and EM receivers.

FIGS. 6A and 6B are graphs illustrating curves of amplitudes and phasesof vertical electric fields acquired using the configuration of FIG. 2in accordance with one or more embodiments of the present disclosure.

FIGS. 7A and 7B are graphs illustrating normalized amplitudes and phasesbased on curves in FIGS. 6A and 6B, in accordance with one or moreembodiments of the present disclosure.

FIG. 8 is a graph showing curves that represent amplitudes of horizontalelectric fields in accordance with one or more embodiments of thepresent disclosure.

FIG. 9 is a block diagram of a computer including software configured inaccordance with one or more embodiments of the present disclosure.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present disclosure. However, it will beunderstood by those skilled in the art that the present disclosure maybe practiced without these details and that numerous variations ormodifications from the described embodiments are possible.

In accordance with some examples, a controlled source electromagnetic(CSEM) surveying technique uses EM receivers and one or more EM sourcesthat are mounted on a tow cable to survey a subterranean structure. Thetow cable is towed by a sea vessel in a body of water. Techniquesaccording to some examples provide the ability to focus energydownwardly into the subterranean structure of interest, and/or reduceunwanted responses from the body of water and/or from the air above thebody of water. In some configurations, the tow cable includes multipleEM sources and multiple EM receivers. In other configurations, the towcable includes a single EM source and multiple EM receivers. In yetanother configuration, multiple tow cables can be used, with each towcable having a combination of one or more EM sources and plural EMreceivers. The multiple tow cables can be towed by a single sea vessel,or by multiple sea vessels.

FIG. 1 illustrates one example configuration that includes a tow cable100 that has plural EM sources 102 and 104, along with plural EMreceivers 106, 108, 110, 112, 114, 116, 118, and 120. The number ofsources and/or receivers depicted in FIG. 1 is provided for purposes ofexample, as different numbers of sources and/or receivers can be used inother implementations. The tow cable 100 is towed by a sea vessel 124 atthe sea surface. The sea vessel 124 has a reel 126 from which the towcable 100 can be unwound for deployment into a body of water 127.

The sea vessel 124 can include a controller 129, which can beimplemented with a computer, to perform data processing on measurementscollected by EM receivers. Alternatively, the controller 129 can belocated remotely, such as at a land location.

The tow cable 100 also includes steering devices 122 arranged at variouspositions along the tow cable. The steering devices 122 can also bereferred to as “steering fish.” The steering devices 122 arecontrollable to steer the tow cable 100 such that the tow cable travelsin a desired direction. Note the number of steering fish employed may bedependent on the length of the tow cable and the desired degree ofaccuracy to which the sensor positions are maintained.

The body of water 127 sits above a sea floor 128, under which is locateda subterranean structure 130. In the example of FIG. 1, a resistive body132 is located at some depth below the sea floor 128, where theresistive body 132 can be a target body of interest, such as ahydrocarbon-bearing reservoir, a fresh water aquifer, a gas injectionzone, a reservoir that contains methanehydrate deposits, a thincarbonate, or a salt-bearing layer, and so forth. Note that pluralresistive bodies may be present at various depths in the subterraneanstructure 130.

A towed marine cable system comprised of the arrangement of the seavessel 124 and the tow cable 100 enables EM measurements taken by the EMreceivers in response to EM signals generated by the EM sources 102 and104. EM signals generated by the EM sources 102 and 104 are affected bystructures within the subterranean structure 130, such as by theresistive body 132. As a result, a signal detected at an EM receivermounted on the tow cable 100 is representative of such effect ongenerated EM signals. Each EM receiver can include a sensor module thathas sensing elements to sense one or more of electric fields, electriccurrents, and magnetic fields. In some cases, the sensing elements canbe arranged to measure electric fields and/or magnetic fields inmultiple different axes, referred to as the x, y, and z axes, where thex and y axes are the horizontal axes (generally parallel to the seafloor 128), and the z axis is the vertical axis (generally parallel tothe depth direction into the subterranean structure 130).

Although reference is made to the horizontal and vertical orientations,it is noted that such reference is made with respect to the arrangementdepicted in the various figures, where the sea floor 128 is assumed tobe flat and has a perfectly horizontal orientation. However, it is notedthat in practical applications, the sea floor 128 will usually have anon-planar surface, and in fact, can have some slope (or can even bevertical). In such cases, the “horizontal” and “vertical” orientationsare intended to refer to relative orientations with respect to thenon-horizontal sea floor.

Each EM source and/or receiver can be a single-component device (to emitor receive an electric or magnetic field) up to a six-component device(with components to emit or receive three electric and three magneticfields), or any multi-component device. The components of each EM sourceor receiver can be excited at a number of frequencies.

The EM sources 102 and 104 can be horizontal electric dipoletransmitters. In other implementations, other types of EM sources can beused, such as a horizontal magnetic dipole transmitter. Also, non-dipoletransmitters can be used in further implementations.

In one example, the towed marine cable system can make EM measurementscontinuously using different combinations of multiple EM sources and EMreceivers on the tow cable 100. Also, the source-receiver combinationscan be optimized to maximize the response at different depths orpositions.

In the arrangement of FIG. 1, note that EM receivers 108, 110, and 112are positioned between EM sources 102 and 104. For EM receivers 108,110, and 112, the dipole moments of the EM sources 102 and 104 can becontrolled such that horizontal electric fields at the EM receivers 108,110, and 112 are substantially cancelled (except for perturbationscaused by two-dimensional or three-dimensional effects). As a result,based on control of the dipole moments of the EM sources 102 and 104,the primary electric field that is detected at each of the EM receivers108, 110, and 112 is a vertical electric field. Note that verticalelectric field data (corresponding to electric fields in the zdirection) is sensitive to resistive bodies at depth, whereas horizontalelectric fields are not sensitive to resistive bodies at depth. Thevertical electric field data can be analyzed for different sourceseparations to characterize conductivity changes with depth. Also, EMreceivers that can measure horizontal EM field data, such as the EMreceivers 106, 114, 116, 118, and 120 outside the region of the towcable 100 between the EM sources 102 and 104, can be used tocharacterize lateral changes.

Thus, in the arrangement of FIG. 1, the different combinations of EMsources and receivers can include a first combination that has EMsources 102, 104, and EM receivers 108, 110, 112 between the EM sources102 and 104. A second combination of EM sources and receivers includesEM sources 102, 104, and EM receivers 106, 114, 116, 118, and 120outside the region of the tow cable between the EM sources 102 and 104.Other combinations of different EM sources and receivers can be defined.The different combinations of sources and receivers are used to collectdifferent measurements that have different sensitivities (with somecombinations being sensitive primarily to presence of resistive bodiesat depth, and other combinations being sensitive to lateral changes).

FIG. 1 illustrates a relatively general configuration, wherecombinations of EM sources and EM receivers in the general configurationcan be set by a user to achieve desired measurements. In a generalsense, the source-receiver configuration, frequency of operation,waveform, and post-acquisition data processing procedures can bedesigned for each survey separately using an optimization process. Theknown geologic parameters in the optimization process are the depth ofsea water (including bathymetry) and its electrical conductivity, thegeometry of the geologic structure beneath the sea floor, and theresistivity of various geologic structures. Note that these parametersare based on pre-survey estimates, or may be in part determined usinginformation from seismic images and other geophysical data.

A computer algorithm (which can be executed by the controller 129) cancompute the EM fields for various source positions, and searches throughcombinations of sources and/or receivers to find the source/receiverconfigurations that produce the largest scattered field at the receiverpositions, or the largest incident field at the location of the targetbody (e.g., body 132 in FIG. 1). A scattered EM field refers to the EMfield scattered from the subterranean structure 130 back up towards theEM receivers. Note in this context, the term “scattered field” canrepresent either the field with the target body minus the field withoutthe target body, or the field with the target body normalized by thefield without the target body. The algorithm may also be designed tofocus energy at different depths, thus providing the ability to combinethe single measurements in different configurations to provide a varietyof different depth sensitivities.

Based on the output of the computer algorithm, the arrangement of EMsources and EM receivers as in FIG. 1 can be defined to achieve desireddepth sensitivities and to provide focused energy at one or more depthsin the subterranean structure 130. Also, the output of the computeralgorithm allows for the frequency of operation, the waveform, andpost-acquisition data processing procedures to be defined.

Although the computer algorithm may employ many sources and a relativelysmall number of receivers on the towed cable, in reality it may be morepower efficient to employ relatively many receivers and few sources.Such power efficiency can be accomplished by using the principle ofreciprocity whereby sources are replaced by receivers with the samepolarization, and vice versa. In other words, an EM source can beconfigured on an EM receiver by disabling the signal driving circuitryand instead using the elements of the EM source to receive signals.

In addition, data may be collected in a single channel (data from allreceivers of the tow cable 100 being transmitted in the single channeland collected) rather than building a complicated source-receiver systemhaving multiple channels. The collected data can be combined into theoptimal configuration in a post-acquisition step.

Another implementation of a tow cable with EM sources and receivers isdepicted in FIG. 2, which shows a tow cable 100A having an array (202)of EM receivers 202A, 202B, 202C, 202D, 202E positioned between threepairs of EM sources, including a first pair 204A, 204B, a second pair206A, 206B, and a third pair 208A, 208B.

The EM receivers 202A-202E can be multi-component EM receivers that areable to measure both electric and magnetic fields. In a differentarrangement, some or all of the EM receivers 202A-202E can besingle-component EM receivers that measure one of electric or magneticfields.

In one implementation, the pairs of EM sources on the tow cable 100A aresuccessively activated to enable measurements to be taken by the EMreceivers in the array 202. For example, the first pair of EM sources204A, 204B can be activated first, while the other EM sources remainoff. Subsequently, the first pair of EM sources 204A, 204B is turnedoff, and the second pair of EM sources 206A, 206B is activated (whilethe third pair of EM sources 208A, 208B remains off). Finally, the firstand second pairs of EM sources 204A, 204B, and 206A, 206B are turnedoff, while the third pair of EM sources 208A, 208B is activated.

Thus, in the arrangement of FIG. 2, three source-receiver combinationsare provided, where a first combination includes EM sources 204A, 204B,and the EM receivers 202A-202E; a second combination includes EM sources206A, 206B, and EM receivers 202A-202E; and the third combinationincludes EM sources 208A, 208B, and EM receivers 202A-202E. Thedifferent pairs of EM sources allow for responses at different depths inthe subterranean structure 130 to be obtained.

Within each pair of EM sources, the dipole moments of the two EM sourcesin the pair are opposed (in other words, the dipole moments are providedin opposite directions such that the phases of the two EM sources are180° out of phase). For example, if the pair of EM sources 204A, 204B isactivated, then the dipole moment of EM source 204A is opposed to thedipole moment of EM source 204B. The electric current is thereby focuseddownwardly into the subterranean structure 130 such that the EM fieldsmeasured by the center EM receiver 202C in the array 202 extend in thevertical direction (z direction). The EM fields in the verticaldirection have maximum sensitivity to the presence of the resistive body132 when no lateral heterogeneity is present (in other words, variationin resistivity is assumed to be in a single direction, the z direction).In the absence of lateral heterogeneity, the EM fields measured at theEM receiver 202C in the center of the array 202 of receivers is entirelyvertical (extends in the z direction). At the center position, thehorizontal electric field (as well as the horizontal magnetic field) iszero. If lateral heterogeneity is present (e.g., there are variations intwo or three dimensions), then perturbations due to such lateralheterogeneity will be detected by horizontal EM fields measured by theEM receivers.

In a shallow water environment, the focusing effect (in the verticaldirection) is enhanced as the electrical current cannot flow upwardlyinto the air above the body of water 127.

In another implementation, instead of activating pairs of EM sources ina sequence, more than two EM sources can be activated at one time in aweighted fashion. Thus, generally, a plurality of EM sources aresimultaneously energized in a weighted manner (e.g., the dipole momentsof two sources are opposed) such that the electric current at the targetlocation (e.g., resistive body 132) is along a predetermined direction(e.g., vertical direction) that provides maximum sensitivity when nolateral heterogeneity is present.

Measurements made by EM receivers (202A, 202B, 202D, 202E) symmetricabout the center receiver 202C can be combined to yield additionalsensitivity to lateral changes. Measurements of the vertical electricfield by the center EM receiver 202C are sensitive to changes withdepth. As a result, by combining measurements of the vertical electricfield with measurements of the horizontal electric and/or magneticfields, sensitivity both to the lateral and depth extent of a subsurfacetarget body can be determined.

Measurements taken using the arrangement of FIG. 2 can also be used tocancel or attenuate unwanted signal components, such as a signalcomponent due to the air-wave effect. The air-wave effect is caused byan EM signal generated by an EM source that follows a path extendingupwardly from the EM source to the sea surface 103, horizontally throughthe air, and then back downwardly through the body of water 127 to areceiver. The signal component due to the air-wave effect (also referredto as a lateral wave) is an undesirable signal component since suchsignal component does not contain information relating to thesubterranean structure 130 being surveyed. The air-wave effect is morepronounced in a shallow water environment. The vertical electric fieldE_(z) that is measured by the center EM receiver 202C is insensitive tothe lateral wave. Also, by using centralized measurements of E_(x) (theelectric field in the x direction along the cable 100A), or by takingthe difference between symmetric measurements of E_(x) about the centerpoint, the lateral wave can be cancelled out. This latter computation isa differentiating technique that is based on taking the difference ofmeasurements by two or more receivers to remove unwanted signals fromthe total measured signals.

In yet another implementation, instead of arranging the EM receiversbetween EM sources, the positions of the EM receivers and EM sources canbe swapped such that the EM sources are located in an array between EMreceivers. Due to reciprocity, the same analysis for the arrangementwhere EM receivers are positioned between EM sources applies.

FIG. 3 shows an alternative configuration that has a tow cable 100B onwhich are mounted a single EM source 302 and multiple EM receivers,identified as R₁₁, R₂₁, R₁₂, R₃₁, R₂₂, and R₃₂. This configuration isreferred to as a receiver-bucking configuration. The receivers are“paired-up” such that in each pair there is a receiver nearer the sourceand a receiver that is farther away from the source. The voltagemeasured by the receiver nearer the source is used to cancel, or“buck-out,” unwanted signals in the far receiver. The unwanted signalcan be the signal generated due to sea water, due to the air-waveeffect, and/or due to near-sea floor sediments. In one example, considerreceiver-pair R₁₁ and R₁₂, as depicted in FIG. 3. If the receiver momentfor the two receivers are identical, then the data can be combined inthe following manner: R1=R₁₂−aR₁₁, where a (a bucking coefficient) is aconstant that is less than unity to force R1 to zero for a predeterminedcondition.

Different combinations of sources and receivers can give different depthpenetrations. Additional bucking measurements can be made by employingR2=R₂₂−bR₂₁ and R3=R₃₂−cR₃₁, where b and c are also buckingcoefficients. Due to the larger offset both from the source, and fromeach other, R2 and R3 progressively sense deeper into the subterraneanstructure.

A variety of techniques can be used to calculate the buckingcoefficients (a,b, and c). One technique uses measurements collectedwith the array in a calibration region of known structure away from thezone of interest. The calibration region can be a region having asubterranean structure similar to the subterranean structure beingsurveyed, except that the subterranean structure of the calibrationregion does not include the resistive body 132. The coefficients arethen calculated such that the bucked-measurements R1, R2, R3 are zero inthis region. Once the bucking coefficients have been calculated based onmeasurements in the calibration region, the tow cable 100B can be movedto the region being surveyed to take measurements. As a post-acquisitionprocessing step (either immediately as the data is being collected orsometime later), the measurement data from the individual receivers canbe combined as described above to obtain R1, R2, R3. Non-zero valueswould then indicate the presence of structure (e.g., resistive body)that is different than that in the calibration region.

A second technique of calculating the bucking coefficients involves anadaptive-numerical processing procedure in which a 1D, 2D, or 3Dnumerical model is created that includes known sea water conductivity aswell as sea floor bathymetry. An average sea floor conductivity is thenassigned to the entire halfspace below the sea floor 128. In apost-acquisition step, the model response is then computed for the knownsource-receiver geometry at each new cable position along the tow cable,and the bucking coefficients are computed to cancel the fields ascalculated from this numerical model. The model allows the buckingcoefficients to be computed without the presence of the resistive body132. Subsequently, the bucked fields measured by the receivers senseconductivity differences between the true subsurface (the subterraneanstructure 130 with the resistive body 132 present) and the uniformseabed conductivity of the model (without any resistive body). Thismethod has the advantage that it incorporates the geometrical changes insource and receiver orientations, distance from sea floor, andbathymetry as the tow cable changes positions.

Alternatively, the bucking can actually be built into the hardware suchthat the moments of the individual receivers are manipulated and theresults summed electronically as the measurements are collected ratherthan digitally at a later time.

Any of the above configurations can employ a multi-streamerconfiguration, such as a dual-streamer configuration shown in FIGS. 4and 5 (where FIG. 4 is a side perspective view and FIG. 5 is a topview). The dual-streamer configuration has two tow cables 100C and 100D,where each tow cable 100C has two EM sources and a number of EMreceivers (similar to the arrangement of EM sources and EM receiversdepicted in FIG. 1). The tow cable 100C includes EM sources 402, 404,and EM receivers, and the tow cable 100D includes EM sources 406, 408,and EM receivers.

The benefit of the multi-streamer configuration is that it allows forcross-line electric sources and measurements. In other words, sources(e.g., sources 402, 406) that are located the same distance behind thesea vessel 124, but on different streamers, can be used to transmitcurrent between the two cables. To achieve the cross-line measurement, asignal source (such as a signal source on the sea vessel) can becontrolled to cause current to pass from one tow cable to the other towcable. Such arrangement causes the dipole moment at each of the pair ofEM sources at the same distance behind the sea vessel, but on differentstreamers, to be perpendicular to the trajectory of the sea vessel.Because the cross-line data is less sensitive to thin resistors atdepth, the cross-line data can be used to better define backgroundresistivities, which can be compared to resistivity identified by inlinedata to enable detection of a subterranean structure. The inline datarefers to data acquired based on passing current through the EM sourcesinline with the tow cables.

FIGS. 6A and 6B are graphs illustrating simulations performed for theconfiguration of FIG. 2. FIG. 6A plots the amplitude of the verticalelectric field (E_(z)) against distance from the center (represented aszero on the horizontal axis) of the array 202 between the EM sources.Curve 600 illustrates the response for a first distance between EMsources (of a first pair), curve 602 illustrates the response for asecond, greater separation between EM sources (of a second pair), andcurve 604 represents the response for a third separation (greater thanthe second separation) between EM sources (of a third pair). A box 606underneath the graph of FIG. 6A indicates the horizontal extent of thetarget body 132.

FIG. 6B plots the phase of E_(z) as a function of distance from thecenter (zero) of the array 202 between EM sources. Curves 610, 612, and614 correspond to the three different separations of EM sourcesdiscussed above for FIG. 6A.

FIGS. 7A and 7B depict curves (620, 622, 624 in FIG. 7A and 630, 632,634 in FIG. 7B) that represent normalized amplitudes and phases,respectively, as functions of distance from center of array. Note thatthe amplitudes and phases are normalized to a calculated amplitude andphase using a model. The curves 620, 622, and 624 represent normalizedamplitudes with increasing offset between sources, and the curves 630,632, and 634 represent normalized phases with increasing offset betweensources. The curves indicate that the system is more sensitive to thetarget body 132 with larger source separations.

FIG. 8 shows curves that represent the E_(x) amplitude as a function ofdistance from the center (zero) of the array 202. Three curves 700, 702,and 704 are depicted to represent different separations between EMsources. Note that the E_(x) response peaks at the edges of the targetbody, and approaches zero near the center, demonstrating how E_(x) ismore sensitive to lateral changes, but is not sensitive to changes withdepth when the configuration of FIG. 2 is employed.

In the various examples discussed above, either frequency domain or timedomain analysis can be performed. In frequency domain analysis, theresponse at different frequencies is determined in a data processingstep. With time domain analysis, however, the transient response ismonitored, in which the EM source(s) are turned on and then subsequentlydeactivated, with the response after deactivation of the EM source(s)monitored to detect for presence of resistive bodies in a subterraneanstructure. A benefit of time domain transient analysis is that distancesbetween sources on the tow cable can be shortened as compared todistances for frequency domain analysis.

An alternative to building a measurement system that focuses the fieldsat a specific target depth is to synthetically focus the data in apost-acquisition processing step. One example method employs alinearized form of the Lipman-Schwinger integral equation governing theelectric or magnetic field, ψ( r, r_(s) ), which is governed by theequation:ψ( r, r _(s) )=ψ( r, r _(s) )+∫d r′g _(b)( r, r′)Q( r′)ψ_(b)( r′, r _(s)),  (Eq. 1)where ψ_(b)( r, r_(s) ) is the background field, Q( r)=σ( r)−σ_(b)( r)is the conductivity of the anomaly (e.g., resistive body 132), and σ( r)is the conductivity distribution in the subterranean structure whileσ_(b)( r) is the conductivity distribution in the background (in theabsence of the anomaly). The location of the observation point (receiverlocation) is denoted by r while that of the source is denoted by r_(s) .The parameter r′ denotes a location inside the anomaly. The functiong_(b)( r, r′) is the Green function of the background medium.

The electric or magnetic field ψ( r, r _(s)) is the measured electric ormagnetic field by EM receivers, which can be receivers in any of thearrangements discussed above or in other arrangements. At a discretenumber of measurements (M) corresponding to differenttransmitter-receiver pairs [{ r, r _(s)}_(i),i=1, . . . , M], thefollowing is derived:

$\begin{matrix}{{{\int{{\mathbb{d}{\overset{\_}{r}}^{\prime}}{K_{i}\left( {\overset{\_}{r}}^{\prime} \right)}{Q\left( {\overset{\_}{r}}^{\prime} \right)}}} = m_{i}},{i = 1},\ldots\mspace{11mu},M,} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$whereK _(i)( r ′)=g _(b)( r _(i) , r ′)ψ_(b)( r ′, r _(si) ),  (Eq. 3)andm _(i)=ψ( r _(i) , r _(si) )−ψ_(b) r _(i) , r _(si) ).  (Eq. 4)

For the purpose of focusing the measurements, the above measurementequation (Eq. 2) is multiplied by w_(i)K*_(i)( r) and summed over allweighted measurements, where {w_(i),i=1, . . . , M} is a set of weightsto be determined later. The following is thus obtained:∫d r′G( r, r ′)Q( r′)=D( r ),  (Eq. 5)where

$\begin{matrix}{{{G\left( {\overset{\_}{r},{\overset{\_}{r}}^{\prime}} \right)} = {{\sum\limits_{i = 1}^{M}{w_{i}{K_{i}\left( {\overset{\_}{r}}^{\prime} \right)}{K_{i}^{*}\left( \overset{\_}{r} \right)}}} = {G^{*}\left( {{\overset{\_}{r}}^{\prime},\overset{\_}{r}} \right)}}},{and}} & \left( {{Eq}.\mspace{14mu} 6} \right) \\{{D\left( \overset{\_}{r} \right)} = {\sum\limits_{i - 1}^{M}{w_{i}m_{i}{{K_{i}^{*}\left( {\overset{\_}{r}}^{\prime} \right)}.}}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

The weights {w_(i),i=1, . . . , M} are chosen such that G( r, r′) ishighly peaked at r′= r, approximating a delta function. In other words:G( r, r′)≈δ( r− r′)  (Eq. 8)

By selecting the weights in this manner, the focused depth is the depthof the anomaly (e.g., resistive body 132). In this case, the followingis obtained:

$\begin{matrix}{{{Q\left( \overset{\_}{r} \right)} \approx {D\left( \overset{\_}{r} \right)}} = {\sum\limits_{i = 1}^{M}{w_{i}m_{i}{{K_{i}^{*}\left( \overset{\_}{r} \right)}.}}}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

Hence, in doing so, the measurements have been focused in software toprovide a direct estimate of Q( r), which is the conductivity of theanomaly (e.g., the resistive body 132). The focusing is done bydesigning the weights {w_(i),i=1, . . . , M} such that:

$\begin{matrix}{{\sum\limits_{i - 1}^{M}{w_{i}{K_{i}\left( {\overset{\_}{r}}^{\prime} \right)}{K_{i}^{*}\left( \overset{\_}{r} \right)}}} \approx {{\delta\left( {\overset{\_}{r} - {\overset{\_}{r}}^{\prime}} \right)}.}} & \left( {{Eq}.\mspace{14mu} 10} \right)\end{matrix}$

FIG. 9 shows a computer 900 in which processing software 902 isexecutable to perform any of the tasks discussed above. The processingsoftware 902 is executable on one or more central processing units(CPUs) 904, which is connected to a storage 906. Results provided by theprocessing software 902 can be output to a display 908, oralternatively, can be communicated over a network to a remote client foroutput.

Data and instructions (of the software) are stored in respective storagedevices (e.g., storage 906 in FIG. 9), which can be implemented as oneor more computer-readable or computer-usable storage media. The storagemedia include different forms of memory including semiconductor memorydevices such as dynamic or static random access memories (DRAMs orSRAMs), erasable and programmable read-only memories (EPROMs),electrically erasable and programmable read-only memories (EEPROMs) andflash memories; magnetic disks such as fixed, floppy and removabledisks; other magnetic media including tape; and optical media such ascompact disks (CDs) or digital video disks (DVDs).

While the present disclosure has been made with respect to a limitednumber of embodiments, those skilled in the art, having the benefit ofthis disclosure, will appreciate numerous modifications and variationstherefrom. It is intended that the appended claims cover suchmodifications and variations as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A marine cable system, comprising: a tow cable; aplurality of electromagnetic (EM) sources coupled to the tow cable; aplurality of EM receivers coupled to the tow cable, wherein at least twoof the EM receivers are arranged between a pair of the EM sources,wherein each of the at least two EM receivers is a multi-componentreceiver to measure electric fields in at least two differentdirections, wherein said system is configured for deployment in a bodyof water to perform marine EM surveying of a subterranean structure. 2.The marine cable system according to claim 1, wherein the EM sources andthe at least two EM receivers are in an arrangement so as tosubstantially cancel a horizontal electric field at one or more of theat least two EM receivers.
 3. A marine cable system, comprising: a towcable; a plurality of electromagnetic (EM) sources coupled to the towcable; a plurality of EM receivers coupled to the tow cable, wherein atleast some of the EM receivers are arranged between a pair of the EMsources, wherein the at least some of the EM receivers arranged betweenthe pair of EM sources comprise an array of EM receivers including acenter EM receiver configured to measure a vertical electric field,resulting in increased sensitivity to changes in depth in a subterraneanstructure, wherein said system is configured for deployment in a body ofwater to perform marine EM surveying of the subterranean structure. 4.The marine cable system according to claim 3, wherein the array of EMreceivers include receivers on either side of the center EM receiverconfigured to measure a horizontal electromagnetic field, resulting inincreased sensitivity to lateral changes in the subterranean structure.5. The marine cable system according to claim 3, wherein the EM sourcesof the pair are to be driven to provide opposing dipole moments, therebyattenuating a horizontal electromagnetic field at the center of thearray.
 6. A marine cable system, comprising: at least one tow cable; aplurality of electromagnetic (EM) sources coupled to the at least onetow cable; a plurality of EM receivers coupled to the at least one towcable; a first streamer by which the plurality of EM sources and theplurality of EM receivers are coupled to the at least one tow cable; anda second streamer by which further EM sources and EM receivers arecoupled to the at least one tow cable, wherein the EM sources on each ofthe two streamers are arranged so as to provide a dipole moment that issubstantially perpendicular to a direction of motion of the twostreamers, wherein said system is configured for deployment in a body ofwater to perform marine EM surveying of a subterranean structure.
 7. Themarine cable system according to claim 3, wherein the pair of EM sourcesand the array of EM receivers are arranged so as to attenuate anair-wave effect.
 8. The marine cable system according to claim 6,wherein at least some of the EM receivers are arranged about a pair ofthe EM sources.
 9. The marine cable system according to claim 6, whereinthe EM receivers of the first streamer and the second streamer acquirecross-line data in response to the EM sources providing said dipolemoment that is substantially perpendicular to the direction of motion ofthe two streamers.
 10. The marine cable system according to claim 6,wherein the EM receivers of the first streamer and the second streameracquire inline data.
 11. The marine cable system according to claim 1,further comprising one or more steering fish that maintain the positionof the EM receivers.
 12. A marine cable system, comprising: a tow cable;a plurality of electromagnetic (EM) sources coupled to the tow cable; aplurality of EM receivers coupled to the tow cable, wherein at leastsome of the EM receivers are arranged between a pair of the EM sources,wherein the at least some of the EM receivers are arranged betweenanother pair of the EM sources, wherein said system is configured fordeployment in a body of water to perform marine EM surveying of asubterranean structure.
 13. The marine cable system according to claim12, wherein the pairs of the EM sources are configured to be activatedin sequence.
 14. The marine cable system according to claim 1, whereinanother subset of the EM receivers is located outside a region of thetow cable between the pair of EM sources.
 15. The marine cable systemaccording to claim 1, wherein the at least two EM receivers arrangedbetween the pair of EM sources comprise an array of EM receiversincluding a particular EM receiver configured to measure a verticalelectric field, resulting in increased sensitivity to changes in depthin the subterranean structure.
 16. The marine cable system according toclaim 15, wherein the EM sources of the pair are configured to causeattenuation of a horizontal electromagnetic field at the particular EMreceiver.
 17. The marine cable system according to claim 16, wherein theattenuation of the horizontal electromagnetic field at the particularreceiver is responsive to driving of the EM sources of the pair toprovide opposing dipole moments.
 18. A marine cable system, comprising:a tow cable; a plurality of electromagnetic (EM) sources coupled to thetow cable; and a plurality of EM receivers coupled to the tow cable,wherein at least two of the EM sources are arranged between a pair ofthe EM receivers, wherein each of the at least two EM sources has acorresponding dipole moment, and wherein said system is configured fordeployment in a body of water to perform marine EM surveying of asubterranean structure.
 19. The marine cable system according to claim18, wherein the at least two EM sources are configured to causeattenuation of a horizontal electromagnetic field at a particular one ofthe EM receivers.
 20. The marine cable system according to claim 19,wherein the attenuation of the horizontal electromagnetic field at theparticular receiver is responsive to driving of the at least two EMsources to provide opposing dipole moments.
 21. The marine cable systemaccording to claim 1, wherein the at least two EM receivers includes anarray of EM receivers, the array including a particular EM receiver andadditional EM receivers on different sides of the particular EMreceiver.